We present a model calculation for the determination of quantum microcanonical average reaction rates for isolated molecules. The model system consists of two coupled electronic states, reactant and product, each with many nuclear motion eigenstates. The reaction rate is evaluated via a resummed perturbation expansion in the electronic coupling. From this we obtain an electronic adiabaticity criterion which depends on the probability to remain at or return to the transition region. We evaluate this survival probability using a global random matrix model for the nuclear Hamiltonian. This leads to an equation for the reaction mte which interpolates between the golden rule electronic crossing rate and the electronically adiabatic rate. We discuss the classical limit and make connection with thermal rate theory.